In a printed-circuit substrate having a copper foil layer on both sides, an electrical connection between a circuit portion of the front side and that of the back side is typically established by forming a through-hole in the desired position and copper-plating the inner wall of the through-hole. The through-hole plating is accomplished by activating the printed-circuit substrate with a palladium-containing bath, and next electrolessly copper-plating the exposed surfaces of the printed-circuit substrate to deposit a thin copper layer, and then performing electrolytic copper plating.
As a technique of electrolytically copper plating a printed-circuit substrate using an insoluble metal electrode, a copper plating method has been proposed in which an insoluble metal anode is separated from a cathode which is the substrate material to be plated, e.g., a printed-circuit substrate, by a diaphragm. Electrolysis is performed in the cathode chamber using an electrolyte solution containing copper ions and an additive, thereby plating the surfaces of the substrate material, including the wall of the through-hole (e.g., see JP-A-1-297884; the term "JP-A" as used herein means an "unexamined published Japanese patent application").
According to this proposed method, the anode solution may be an electrolyte which differs from the plating or cathode solution and contains no copper ions. That is, entirely different types of electrolytes may be used as the anode and cathode solutions, respectively. In this case, however, the use of different electrolytes results in an increased osmotic pressure over the diaphragm, and this causes a difference in liquid level to development in a relatively short period of time between the anode and cathode solutions and poses other problems. In order to remove such problems, complicated equipment must be used which, for example, includes two circulating pumps installed separately for the two electrolyte solutions. This circumstance leads to increased equipment costs.
On the other hand, in the case of using the same type of plating solution for both the anode and cathode solutions, a maintenance problem arises. Specifically, in order to prevent the additive contained in the plating solution from being consumed on the insoluble metal anode, the anode chamber is first filled with the electrolyte and feeding of the electrolyte to the anode chamber from the cathode chamber is then suspended, and thereafter replenishment is done intermittently according to need.
Although electrolyte replenishment of the anode chamber may be attained by means of osmosis from the cathode chamber to anode chamber through the diaphragm, this is not a reliable method over an extended period of operating time because clogging of the diaphragm and other troubles may occur.
There is no known permanent expedient which prevents consumption of the additive in the electrolyte solution by the insoluble anode and also corrects for a reduction in the amount of anode solution due to the electrolysis or evaporation of water in the anode chamber.